The quest for phosphate bioisosters over the last several decades included the synthesis of phosphonates, α-halo (e.g. difluoromethyl) phosphonates (Blackburn, 1981; Blackburn et al., 1981 and 1987), phosphorothioates (Nahorski and Potter, 1989; Eckstein, 1983, 1985, and 2000) and boranophosphate analogues (Sood et al., 1990; Summers et al., 1998; Shaw et al., 1993 and 2000).
Phosphates and phosphate-containing molecules play a major role in numerous biological systems (Westheimer, 1987 and 1992). However, the unwanted lability of the ester P—O bond has promoted the search for suitable bioisosters, phosphate analogues, which retain biological activity but possess diminished lability. The search for bioisosters was initiated by the need to produce phosphate probes for various studies, such as probing stereochemical requirements of enzymes (Roumaniuk and Eckstein, 1981; Conolly and Eckstein, 1982). In addition, phosphate bioisosters have been developed for improving the pharmacological effects of nucleotide-based drugs, e.g. anti-sense agents (Agrawal, 1999; Stein, 1996).
A widely used isoster of phosphate is phosphorothioate and its analogues, proposed in the pioneering work of Eclkstein et al. (Nahorski and Potter,1989; Eckstein, 1983, 1985, and 2000). In these analogues, the nonbridging oxygen atom is replaced by a sulfur atom. Other chemical modifications of the phosphate moiety include the substitution of the labile phosphate ester oxygen atom by carbon or nitrogen atom, to give phosphonates and phosphoramidate analogues, respectively (Engel, 1977).
During the last decade, pioneering studies by Spielvogel and Ramsay-Shaw have proposed boranophosphate analogues 1 as bioisosters of natural nucleotides (Sood et al., 1990; Shaw et al., 2000) and as important tools for biochemists (Rait et al., 1999; Zhang et al., 1997; Porter et al., 1997).

This new class of boron modified nucleotides, that mimic phosphodiesters, phosphorothioates, and methyl phosphonates, was designed for use as potential therapeutic and diagnostic agents. These nucleoside boranophosphates, or borane phosphonates, have a borane moiety (BH3) in replacement of one of the nonbridging oxygen atoms in the phosphate diester moiety. The BH3 group maintains the negative charge of a phosphate, but it does not form classical H-bonds and does not coordinate with metal ions. This modification imparts unique characteristics to boranophosphate nucleotides and nucleic acids. The boranophosphate can be considered as a “hybrid” of three well-studied types of modified phosphates, namely, normal phosphate, phosphorothioate, and non-ionic methylphosphonate. The BH3 group in the boranophosphates is isoelectronic with oxygen (O) in the normal phosphates, and isolobal (pseudo-isoelectronic) with sulfur (S) in phosphorothioates. The BH3 group is isosteric with the CH3 group in the methylphosphonates. Boranophosphates would be expected to share a number of chemical and biochemical properties with phosphorothioate and methylphosphonate analogs. Thus, boranophosphate analogues have a different charge distribution and polarity than the corresponding natural nucleotides (Shaw et al., 1993).
This emerging field of novel nucleotide bioisosters has expanded significantly and has provided many important applications of the boranophosphate analogues. For instance, non-terminal P-boronated nucleotides, existing as a pair of diastereoisomers, have been used as stereochernical probes to elucidate enzymatic catalysis (Sergueeva et al., 2000). Oligodeoxyribonucleotides bearing boranophosphate linkages have been used for polyrnerase chain reaction (PCR) sequencing and DNA diagnostics (He et al., 1999; Porter et al., 1997), and boranophosphate nucleotides have been found to be highly potent and stable P2Y-receptor agonists (Nahum et al., 2002). Oligonucleotides bearing boranophosphate linkages have also been considered as potentially useful anti-sense agents (Summers and Shaw, 2001). These analogues were also tested for the treatment of cancer as carriers of 10B isotope in boron neutron capture therapy (Spielvogel et al., 1992). However, despite the extensive study of related boranophosphate nucleotide/oligonucleotide analogues, the exploration of the parent inorganic boranophosphate has not been reported.
The various potential applications of a phosphate isoster, together with the limitations of the currently available isosters, justify the continued search for the perfect inorganic phosphate mimic.